This invention relates to a direct injection engine which is provided with an injector for injecting fuel directly into a combustion chamber and produces stratified charge combustion by injecting the fuel during each compression stroke in a low-speed, low-load engine operating range, for instance.
Direct injection engines are commonly known in the prior art. In one example, a conventional direct injection engine comprises a piston having a cavity formed in its top surface, an injector for injecting fuel directly into a combustion chamber, and a spark plug located in the vicinity of the cavity, wherein the injector injects the fuel toward the cavity during a compression stroke in a low-speed, low-load operating range, for instance, so that a mixture is locally distributed around the spark plug, causing stratified charge combustion to occur. In this direct injection engine, it is possible to significantly increase the average air-fuel ratio (leaner mixture) due to the stratified charge combustion and thereby achieve improved fuel economy.
In the aforementioned conventional direct injection engine, it is desirable to expand the operating range in which the stratified charge combustion takes place as much as possible in order to increase fuel economy improvement effect. However, there arises a problem that the mixture becomes overrich around the spark plug if the amount of injected fuel increases in a relatively high-load region within the stratified charge combustion range.
More particularly, a condition in which a mixture having an appropriate air-fuel ratio is locally distributed around the spark plug is obtained when the amount of injected fuel is small because a fuel spray injected from the injector during the compression stroke is trapped by the cavity in the piston, concentrating the mixture around the spark plug. When the amount of injected fuel increases, however, the fuel is too concentrated around the spark plug and this produces an overrich mixture around the spark plug.
To avoid this problem, U.S. Pat. No. 5,553,588 proposes an engine comprising a piston having a cavity formed in its top surface, a spark plug an injector, and means for producing a swirl within the cavity, wherein the injector is installed with its nozzle opening oriented at an oblique angle to the direction of a central axis of the injector so that the fuel injecting direction points to the upstream side of the spark plug with respect to the rotating direction of the swirl.
According to this engine, a fuel spray injected from the injector is diverted from the direction of the spark plug and reaches an area of a cavity wall on the upstream side of the spark plug with respect to the rotating direction of the swirl, where the fuel spray is caused to spread to a certain extent by the swirl. The fuel spray flows downstream along the swirl while achieving a flammability limit air-fuel ratio, and passes close to the spark plug. Therefore, compared to other conventional engines in which the fuel is concentrated around the spark plug, the engine of U.S. Pat. No. 5,553,588 makes it possible to prevent excessive richness of the mixture to a certain degree and expand the operating range in which the stratified charge combustion can take place.
Even when the nozzle opening of the injector is always oriented to the upstream side of the spark plug to divert the fuel spray away from the direction of the spark plug, this arrangement alone is not sufficient, because spreading of the mixture would become insufficient and an overrich mixture mass could be formed locally when the amount of injected fuel is increased to a certain degree. Thus, there is left room for improvements in the above-described arrangement.